franklinmanklin
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- Michigan
If I have 30A, 2-pole breaker feeding a 10 Horsepower, static phase converter that feeds a 21A, 230V, 3-phase motor; what is the current at the breaker on each of the two poles?
Current Rating on the Breaker
- Thread starter franklinmanklin
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- Electrical Engineer
No way to know an absolute number, because the LOAD determines the current drawn by your motor, not the rating. The 21A FLC just tells you the MAXIMUM current the motor can safely draw, not what it ACTUALLY draws.
But whatever current is drawn by the motor based on the load it is connected to, will be increased by a factor of 1.732 on the input side of your phase converter, plus any inherent losses in the converter (which we also don't know). So of your 3 phase motor current is 10A, the current drawn from your 1 phase supply will be 17.32A, but only if your converter is 100% efficient, which never happens. If you assume worst case scenario, that the converter is 80% efficient, then the current drawn will be 21.65A (17.32/.8).
Hope that helped you.
But whatever current is drawn by the motor based on the load it is connected to, will be increased by a factor of 1.732 on the input side of your phase converter, plus any inherent losses in the converter (which we also don't know). So of your 3 phase motor current is 10A, the current drawn from your 1 phase supply will be 17.32A, but only if your converter is 100% efficient, which never happens. If you assume worst case scenario, that the converter is 80% efficient, then the current drawn will be 21.65A (17.32/.8).
Hope that helped you.
franklinmanklin
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- Michigan
Wow. I wonder if it would have been better to get a transformer and run it at 460V. I also read on the eBay seller's page that the output isn't truly 3-phase power and that it runs at reduced horsepower. When I checked voltage I got 240V from A-B and B-C but nothing from C-A. Each phase to ground was 120V. I plan on amprobing the wires to see for sure. I also see some guys online recommend a VFD or run a 3-phase motor with a single-phase motor and couple them. BTW, this is for a lathe.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Something is wrong here.
Do you have 3 phase input, or single phase? Because if you have a "C", that implies 3 phase, but having 0V from A-C would be impossible. Where are you measuring at?
I think perhaps you have single phase 240V from a normal US 240/120V service, and someone has wired up 2 poles of a 3 pole device where the lines come into A and B, then A loops back onto C. So you are reading A-B when reading B-C as well, but because A and C are the same leg, you read zero. it
s a trick that Europeans must do on some protective devices, because they are not made for single phase systems as we use them here in the US, so they have to "trick" the devices into thinking they are seeing 3 phases. Don't let that confuse you.
If you have a single phase service and a 3 phase motor, you must somehow get a device that converts single phase to 3 phase, or replace the motor with a single phase motor. A Static Phase Converter is one way to accomplish that conversion, as is a VFD. The SPC will not be anywhere near as efficient as the VFD.
BTW, running it at 460V would make no difference whatsoever, other than complicate things if you want to use a VFD, because you need to add a transformer and the protection for it, then some 480V VFDs will not allow you to use single phase input. That's another wrinkle that's not worth discussing here other than to just avoid it; going to 460V buys you nothing at all.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
I wonder if the so called 'static phase converter' is actually the set of components that would ordinarily be used to get a rotary phase converter spinning.
A rotary phase converter is just a three phase induction motor. Two legs are connected to your supply, and when the motor is spinning the third leg is generated by the motor. To get the motor spinning some sort of phase shift network is used, such as capacitors. Usually this network gets disconnected by a contactor.
This same sort of system could be used to spin a motor; apply single phase two two legs plus a phase shifted third leg, get the motor going, and then disconnect the motor, leaving it spinning 'single phased'.
If you have a link to the auction page, perhaps we can figure out more.
-Jon
A rotary phase converter is just a three phase induction motor. Two legs are connected to your supply, and when the motor is spinning the third leg is generated by the motor. To get the motor spinning some sort of phase shift network is used, such as capacitors. Usually this network gets disconnected by a contactor.
This same sort of system could be used to spin a motor; apply single phase two two legs plus a phase shifted third leg, get the motor going, and then disconnect the motor, leaving it spinning 'single phased'.
If you have a link to the auction page, perhaps we can figure out more.
-Jon
Tony S
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Steinmetz system
Carultch
Senior Member
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- Massachusetts
Here is a couple of fake acronyms for you, to help remember the terms:
FLC = "full load code"
FLA = "full load actual"
FLC is the full load current that the NEC prescribes for a motor of a given mechanical output power rating. An upper limit of current-to-horsepower in common motors that existed at the time the table was constructed.
FLC is the full load amperes that the datasheet of a motor indicates, based on manufacturer's test results.
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- Placerville, CA, USA
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- Retired PV System Designer
I think the last paragraph should refer to FLA.
franklinmanklin
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- Michigan
Static Phase Converter Auction
Static Phase Converter Auction
http://www.ebay.com/itm/HD-3-10-Hp-...231?pt=LH_DefaultDomain_0&hash=item41862ee417
I think a couple of guys missed my first post. At any rate, I was trying to figure out the load at the single phase breaker without using a meter. Dividing by the square root of three is not really black and white due to the absence of a true third phase.
Static Phase Converter Auction
http://www.ebay.com/itm/HD-3-10-Hp-...231?pt=LH_DefaultDomain_0&hash=item41862ee417
I think a couple of guys missed my first post. At any rate, I was trying to figure out the load at the single phase breaker without using a meter. Dividing by the square root of three is not really black and white due to the absence of a true third phase.
I have gone through all the replies, and to my knowledge, this replies suits bets to the post and is to the point, illustrating. thanks
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